This invention relates generally to methods for beneficiation of industrial minerals, and more specifically relates to an improved method for dewatering industrial minerals of very fine particle size, such as kaolin clays.
In the course of processing various crude industrial minerals such as kaolin clays, the crude materials are commonly subjected to a series of steps intended to remove undesired impurities such as discolorants, so as to ultimately provide a refined product which is susceptible to the particular use for which it is intended. In the instance, for example, of kaolins, such beneficiation may in some instances be accomplished by dry processing techniques, as for example by air flotation. More commonly, however, especially where the impurities or contaminants to be removed are in some manner chemically combined or intimately associated with the mineral, wet processes of various types are employed. These include such well-known processes as froth flotation, reductive and oxidative bleaching, and high intensity magnetic separation. This is in addition to wet classification, which can also be regarded as a beneficiation step achieving some of the aforementioned objectives.
Thus, in a typical procedure wherein the initial starting material is a crude sedimentary kaolin clay, including from about 1-2% titania, the said crude may be subjected to a separation process to reduce the titania content to below 0.8% by weight. More generally, the titania will be reduced to the range of from about 0.2 to 0.8. Where a froth flotation process is used for such purposes, the clay may be initially blunged and conditioned by forming an aqueous alkaline dispersion of the clay (pH adjusted to about 7-10 with ammonium hydroxide). The dispersion may include as a deflocculating agent various compounds known to be useful for such purposes, such as sodium silicate. Other useful agents include a water-soluble salt of a polyacrylic acid or polymethacrylic acid preferably having an average molecular weight in the range of from about 500-10,000. Oleic acid or other collector agent is added during the conditioning process. Reference may be had to U.S. Pat. No. 3,974,067 for further details of flotation procedures which may be utilized. Further aspects of flotation treatment of the aforementioned type can be found in numerous places in the prior art, including in Cundy, U.S. Pat. No. 3,450,257, and in U.S. Pat. Nos. 2,990,958 and 3,138,550.
The purpose of the froth flotation in the foregoing sequence is to remove titania; and accordingly other techniques can be utilized in place of or to supplement flotation, including by passing the slurry in relatively dilute form and while the clay is dispersed (typically at about 30% solids) through a high intensity magnetic field, e.g. via a magnetic separator of the type disclosed in Marston, U.S. Pat. No. 3,627,678. Such device comprises a canister packed with stainless steel wool at which enveloping magnets are capable of providing a high intensity field of 12 kilogauss or higher. Froth flotation may be combined with magnetic separation to achieve additional effects, see for example U.S. Pat. No. 3,974,067 to Allen & J. Knott.
A further commonly used method for improving the brightness and whiteness of kaolin clays involves chemical bleaching. In this connection, it is noted that one of the principal sources of discoloring contaminants in the crude clay takes the form of insoluble oxides of iron. Thus, a common bleaching technique for removing the said contaminants, involves forming the clay into an aqueous slurry, acidifying the slurry to a pH of the order of 3.0 to 4.0 and adding a slurry soluble salt of hydrosulfurous acid. The general objective of this operation is to provide the S.sub.2 O.sub.4 =ion which acts as a reductive bleaching agent. Such ion functions to reduce the ferric compounds present in the slurry to ferrous form, the latter being readily soluble and therefore removable by subsequent washing, dewatering and filtering operations.
A still further type of beneficiation treatment applicable to kaolin crudes involves high temperature calcination. It is useful here to point out that those skilled in the art of kaolin processing products draw a relatively sharp distinction between so-called calcined clays and clays which have not been subjected to calcination and are usually referred to as "hydrous" clays. With respect to such terminology, it is noted that the prior art literature, including numerous of the prior art patents relating to the field of kaolin products and processing, often uses the term "hydrous" to refer to a kaolin which has not been subjected to calcination--more specifically, which has not been subjected to temperatures above about 450.degree. C., which temperatures serve to impair the basic crystal structure of kaolin. These so-called "hydrous" clays may have been produced from crude kaolins, which have been subjected to beneficiation as, for example, to froth flotation, to magnetic separation, to mechanical delamination, grinding, or similar comminution, but not to the mentioned heating as would impair the crystal structure.
In an accurate technical sense, the description of these materials as "hydrous" is, however, incorrect. More specifically, there is no molecular water actually present in the kaolinite structure. Thus, although the structure can be (and often is) arbitrarily written in the form 2 H.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2, it is now well-known that kaolinite is an aluminum hydroxide silicate of approximate composition Al.sub.2 (OH).sub.4 Si.sub.2 O.sub.5 (which equates to the hydrated formula just cited). Once the kaolin is subjected to calcination, which, for the purposes of this specification means being subjected to heating of 450.degree. C. or higher for a period which eliminates the hydroxyl groups, the crystalline structure of the kaolinite is destroyed. Therefore, such material, having been thus calcined, cannot correctly be referred to as a "kaolin". Accordingly, it should be appreciated that henceforth in this specification, when the term "kaolin" or "kaolinite" is utilized, such term necessarily implies that the original structure of the material is intact. Thus, the term "kaolin" as used herein, can be considered to be equivalent to the technically inaccurate (but oft-occurring) prior art usage, "hydrous kaolin" or sometimes simply "hydrous clay".
Detailed discussions of calcined clays and their method of preparation may be found in numerous prior art patents. Particular reference may be made in this connection to U.S. Pat. No. 3,014,836 to Proctor, Jr.; U.S. Pat. No. 3,586,523 to Fanselow et al; and to A.D. McConnell et al. U.S. Pat. No. 4,381,948. The procedures set forth for producing a calcined clay is detailed in the said McConnell et al patent and provides a product substantially corresponding to the commercially available product ALPHATEX.RTM. of the present assignee ECC America Inc. In the said procedure, which is exemplary of modern practice in the calcined clay art, the crude kaolin clay is blunged and dispersed to form an aqueous dispersion. The blunged and dispersed aqueous slurry is subjected to a particle size classification from which there is recovered a fine fraction slurry of the clay. Following this, the clay may be dewatered by being flocculated and filtered, redispersed as a slurry and dried; or the classified slurry may be dewatered by directly drying, for example by spray drying.
As is discussed in Fanselow et al and elsewhere, the calcined clay process as same has been outlined, can be supplemented by use of additional beneficiation steps such as those previously discussed, i.e. froth flotation, high intensity magnetic separation, and the like.
It will be evident from the foregoing that whether one is considering the processing of so-called hydrous clays or of calcined clays, at various points in the processing of same, dewatering by filtration is a common and necessary step. The predominant practice in the kaolin industry calls for such filtration to be accomplished by rotary vacuum filters (RVF), although other instrumentalities are also used as will be further discussed. Dewatering of kaolin slurries by use of such apparatus is normally accomplished in an acid (3.0-3.5 pH) flocculated condition. Most commonly, the dewatering process usually follows bleaching, and entails heating to approximately 130.degree. F. and filtering using a rotary vacuum drum filter. Typically this technique produces a 56-62% solids product cake, and is regarded as the industry standard.
Dewatering by use of plate and frame presses, is also accomplished in an acid (3.0-3.5 pH) flocculated low solids (20-30%) condition. This technique produces a 70-72% solids product cake, but is not generally considered cost effective. Automation in recent years has somewhat offset the economic disadvantage of this type of apparatus.
In a series of patents including e.g. U.S. Nos. 3,753,498, 3,753,499, and 3,782,554, assigned to ECC International Limited at St. Austell, Cornwall, England, tube presses are described, which are also useful when filtering kaolins and similar very fine particle size minerals. Dewatering is again accomplished in an acid (3.0-3.5 pH) flocculated low solids condition. The technique has the advantage of producing a 75% solids product cake, but is unfortunately maintenance and cost intensive.
One of the known exceptions to the inability of prior art filtering techniques to effectively filter very fine particle size minerals such as kaolins having particle size distribution (PSD) such that 50% or more of same by weight are of less than 0.5 .mu.m, involves use of the so-called electrically augmented vacuum filter ("EAVF"). Reference may be had in this connection to such use in Mixon. Jr., U.S. Pat. No. 4,246,039. Use of an EAVF enables filtering of dispersed (7.0-9.0 pH) high solids (55-60%) feed slurries; and the said filter is also capable of producing an 80-85% solids product cake. The EAVF technology is such that flocced kaolins cannot be effectively filtered. While it would be thought because of these advantages that the said filtering technique would have come into widespread commercial use, such is decidedly not the case. This is explained by the complexity of the device (which is based upon the principle of providing an electric field which appropriately interacts with a membrane), and the attendant high costs of maintaining and operating such device. Simply stated, the devices have not proved reliable or economically feasible for widespread application.
A further technique which has found application to dewatering of clays, is simple thermal evaporation. According to this technology, 60% solids filtered product can be thermally evaporated until the slurry reaches 70% solids. This technology is of specialized interest in application, but cannot meet the variety of needs and conditions for use required in industry such as the kaolin processing industry.
A still further technique for dewatering involves the use of centrifuges. These systems can be very effective for dewatering especially with feeds in the 20-30% range, and products up to the 50% range. When these limits are reached, the slurry viscosity and mechanical speed constraints, reduce cost effectiveness and performance to unacceptably low levels.
For some years the concept of ceramic filtration has been known as a method for dewatering a range of materials including minerals, and such filtration techniques are in commercial use at various points in the world for the dewatering and filtering of relatively coarse minerals. Certain recent developments in the field of ceramic filtration have provided ceramic filter constructions which have proved very effective in the filtration of relatively coarse sized minerals. Reference may especially be had in this connection to international applications WO88/06480, WO88/07402, and WO88/07887. The ceramic filtering elements described in these applications are characterized by an underlying porous support layer, and an overlying porous filtering layer. The ratio of the mean pore size in the support layer to that of the filter layer is in the range of about 2 to 50. The pore size in the support layer is of the order of 1-3 .mu.m and that of the filter layer is in the range of from about 4-10 .mu.m. The pores are relatively uniform--i.e., there is a very narrow distribution (within the ranges indicated), so that one may operate at a determined bubble point with assurance that negligible air will pass through the filter during filtration, which may be accomplished at a very high efficiency. Further details respecting composition and preparation of these ceramic filter materials is set forth in said publications, the entire contents of which are hereby incorporated by reference.
Heretofore, the use of these materials has been limited to relatively coarse particle minerals, having a size range of approximately 1 .mu.m or (in general) much higher. In the case of minerals such as kaolins, it has heretofore proved impractical on any commercial scale to successfully effect filtration of such materials where the average particle size is substantially below about 1 .mu.m where substantial quantities of the particles are &lt;1/2 .mu.m, and where the slurry from which said kaolin is to be filtered contains the kaolin is a dispersed or fully dispersed state. This is indeed the reason why the great bulk of filtration practiced in the kaolin industry, requires the use of flocculation as a precedent step to filtration. Consideration of the prior art and patents and the like will amply demonstrate this point.
Aside from the fact that flocculation introduces a complex and additional step into kaolin processing, it also has the effect of adding further undesired chemical species into the slurry, i.e., the flocculation is usually accomplished by acidification, e.g. with the addition of sulfuric acid or other acid-introducing species, which further complicates the process chemistry due to the need for later neutralization. The result of the latter can be the creation of fluxing agents, unless a great degree of washing is used, which indeed is one of the further consequences of flocculation and conventional filtering. (Fluxing agents are of great concern where the kaolin being processed is intended as a calciner feed.) It will be evident that were it possible to effectively filter to high solids and at commercially acceptable rates by simple and effective means, that a vast improvement in kaolin processing would be enabled.
In accordance with the foregoing, it may it be regarded as an object of the present invention to provide a method, whereby slurries of fine particle sized minerals, including especially kaolin clays, may be effectively and rapidly filtered, even where same are in a partially or fully dispersed state.
It is a further object of the invention, to provide a method of the foregoing character, wherein filtration is effected without the need for addition of flocculating agents; and where further, such filtration can be accomplished at any desired pH, including neutral pH.